This application is a continuation of 10/090,667, filed Mar. 5, 2002, now U.S. Pat. No. 6,599,364.
1. Field of the Invention
The present invention relates to an apparatus for applying liquid or pasty application medium, in particular aqueous pigment suspension, to one or both sides of a moving material web, especially to a paper or board web, during its production and/or finishing
2. Description of the Related Art
Machines for the production and/or finishing of a material web, in particular a paper or board web, have widths of up to 10 m and more nowadays. In the case where the material web is coated by way of an application medium curtain, a uniform application reaching over the entire web width is to be ensured. This requirement is more difficult to realize the faster the material webs run in the machines and, in addition, also depends on the quality of the material web and the application medium.
Coating with a “curtain coater”, that is to say an application medium applicator unit, is carried out without any excess of medium. The amount of medium applied is only that which is also intended to remain on the material web. The film applied is therefore very thin.
Depending on the quality of the material web used, the formation of microscopically uncovered points therefore occurs (“micro-skip coating”), the aforementioned very thin liquid film tearing at locations with projecting paper fibers, roughness or pores. At these locations, which are different in point terms, the homogeneous contact between medium and substrate (material web) is lacking.
In order to counter this problem, one solution for the electrostatic charging of the material web and/or the application medium and/or the applicator unit has been proposed in DE 100 12 256. DE 100 57 731 is likewise concerned, among other things, with the problem of the more uniform wetting of the material web to be coated by a curtain applicator unit. In the case of this solution, the arrangement of a vacuum generating device in the form of a suction box is provided in the pocket between the application medium curtain and the material web. In the case of direct application to the material web, the vacuum is intended to promote anchoring of the application medium on the material web. In order to increase its efficiency, the aforementioned suction box is equipped with doctor elements, which are set against the material web, on the inlet side and/or the outlet side (in relation to the direction of the moving material web).
What is needed in the art is a suitable apparatus to improve further the adhesion to a moving material web of the application medium curtain produced by a curtain applicator unit, and to ensure still more uniform wetting of the material web surface.